The invention relates to a process for production of polycrystalline silicon.
Polycrystalline silicon (polysilicon for short) serves as starting material in the production of monocrystalline silicon by means of crucible pulling (Czochralski or CZ process) or by means of zone melting (Floatzone or FZ process). This monocrystalline silicon is separated into wafers and, after a multitude of mechanical, chemical and chemomechanical processing operations, used in the semiconductor industry for manufacture of electronic components (chips).
More particularly, however, polycrystalline silicon is required to an increased extent for production of mono- or polycrystalline silicon by means of pulling or casting processes, this mono- or polycrystalline silicon serving for manufacture of solar cells for photovoltaics.
The polycrystalline silicon is typically produced by means of the Siemens process. In this process, thin filament rods (“thin rods”) of silicon are heated by direct passage of current in a bell-shaped reactor (“Siemens reactor”), and a reaction gas comprising a silicon-containing component and hydrogen is introduced.
The silicon-containing component of the reaction gas is generally monosilane or a halosilane of the general composition SiHnX4-n (n=0, 1, 2, 3; X=Cl, Br, I). It is preferably a chlorosilane or chlorosilane mixture, more preferably trichlorosilane. Predominantly SiH4 or SiHCl3 (trichlorosilane, TCS) is used in a mixture with hydrogen.
EP 2 077 252 A2 describes the typical structure of a reactor type used in the production of polysilicon.
The reactor base is provided with electrodes which accommodate the thin rods onto which silicon is deposited during the growth operation, and which thus grow to become the desired rods of polysilicon. Typically, in each case two thin rods are connected with a bridge to give a thin rod pair, which forms a circuit via the electrodes and via external devices, and this serves to heat the rod pairs to a particular temperature.
The rod temperature is typically measured with radiation pyrometers at the surfaces of the vertical rods.
The rod temperature is typically set by regulating the electrical power, either in a fixed manner or else as a function of the rod diameter.
In addition, the reactor base is additionally provided with nozzles which supply the reactor with fresh gas. The offgas is conducted back out of the reaction chamber via orifices.
The amount of reaction gases supplied is typically varied as a function of the rod diameter, i.e. generally increases with increasing rod diameter.
At the heated rods and the bridge, high-purity polysilicon is deposited, as a result of which the rod diameter grows with time (CVD=chemical vapor deposition/gas phase deposition).
The resulting polysilicon rods, after the deposition operation has ended, have to be processed to give lumps and chips, unless they are intended for the production of single crystals by the FZ process.
In the latter case, the polysilicon rods are used to produce, by zone melting, monocrystalline silicon ingots which are processed further at a later stage to give silicon wafers.
In order to produce silicon chunks for CZ or solar, the rods are mechanically comminuted with tools such as hammers, crushers or mills and then classified by size. The size of the silicon pieces ranges from about 1 mm up to pieces of 150 mm or more. The shape of the pieces should typically not deviate too greatly from the spherical form.
All known processes for deposition of polysilicon which are based on the Siemens process have disadvantages with regard to the product quality achieved and the economic viability of the production process.
Disadvantages with regard to the product quality are especially an often observed axial variation in the rod diameter, and in some cases poor surface characteristics of the rods.
The processes often require an elevated level of energy.
In some cases, the rods fall over in the reactor.
Finally, silicon dust forms in some processes.
In some processes, there is overheating and, in the worst case, even melting of the silicon carrier body (rods and bridges).
DE 29 12 661A1 describes a process for producing of polycrystalline silicon, in which partly liquid trichlorosilane is introduced into the reactor chamber by means of a specific nozzle (two-jet nozzle). This is intended to increase the proportion of trichlorosilane in the reaction gas and ultimately to achieve a higher output. It has been found here, however, that the specific energy consumption is much too high.
In EP 2 077 252 A2, it is considered to be advantageous from a process technology point of view to switch nozzles for the reaction gas supply on and off during the deposition process. For this purpose, the proportion of closed nozzles is regulated as a function of process time or rod diameter.
The aim of this measure is, with growing rod diameter, to ensure optimal gas supply of all rods—especially within the upper range.
EP 2 067 744 A2 discloses a production process for polycrystalline silicon, in which the inflow rate of the reaction gas by which silicon is deposited, after a first stabilization step, is increased first significantly and then more slowly in order to improve the supply of the thin rods with reaction gas, and is then reduced in the growth step in order to ensure efficient deposition. It is emphasized that merely the supply with reaction gas is regulated, and hence no modifications whatsoever to the reactors are required.
However, the processes described in EP 2 077 252 A2 and in EP 2 067 744 A2 exhibit an increased number of rods falling over. This is probably connected to the abrupt changes in the inflow rates of the reaction gas.
The length of the thin rods used may be several meters (about 2-3 m is customary). When falling over, rods can also knock over other adjacent rods.
This causes considerable economic damage, especially when silicon rods contaminated in this way have to be cleaned in a complex manner, or even the reactor is damaged when the rods fall over.
If this occurs before the end of the deposition process, the deposition operation has to be stopped immediately in order to recover the rods which have fallen over. This has direct effects on the economic viability of the corresponding plant. The more rods are present in the reactor, the greater the economic damage. On the other hand, high economic viability in normal operation is coupled directly to a high number of rods in the reactor.
A further disadvantage of the process disclosed in EP 2 067 744 A2 is that it is apparently impossible to achieve a constant geometry or morphology over the entire rod length and at the same time a sufficiently high deposition rate.
The geometry of a silicon rod during the deposition process corresponds ideally to a cylinder of growing radius. Deviations from this cylinder form can cause disruption to the process. In the most unfavorable case, rods can fuse together and stop the deposition operation. With regard to the deviation mentioned, if the result is a conical shape narrowing in the downward direction, there is again also an increased risk that the rods will fall over due to the less favorable weight distribution.
The requirements on polysilicon rods which are to be used later for the float zone process are particularly strict. Before use, the rods are ground to a nominal diameter with a round shape. Any exceedance of the target diameter means that an increased amount of material is removed in the round grinding and hence valuable silicon is lost. If the diameter is ever lower than the target diameter, on the other hand, the length of the rod piece to be used is reduced and hence the economic viability of the target product is worsened.
Rods of polysilicon can be described not only in terms of length and diameter but also by means of further parameters: the nature of the surface of the rod may be different. The rod may have a cauliflower-like surface. The rod may, however, also have a substantially smooth surface. The overall properties of the surface of the rod shall be referred to hereinafter by the term “morphology”.
It is known that primarily a high mean deposition rate is crucial for a high productivity and hence for the economic viability of the process. There is therefore an effort to maximize the deposition rate if possible. However, a higher deposition rate usually requires process conditions which have an adverse effect, for example, on the morphology.
DE 102 007 047 210 A1 discloses a process which leads to polysilicon rods with advantageous flexural strength. Moreover, the specific energy consumption in this process is particularly low. In terms of process technology, a maximum value of the flow rate of the chlorosilane mixture is attained within fewer than 30 hours, preferably within fewer than 5 hours, the temperature at the underside of the bridge being between 1300° C. and 1413° C.
However, a problem is that the temperature in the interior of the bridge can be higher than the temperature at the bridge surface, which is maintained between 1300° C. and 1413° C. according to DE 102 007 047 210 A1.
The temperature is regulated by the electrical current in rod and bridge. In order to be able to maintain the temperature in the event of cooling of the bridge surface by inflowing gas, the electrical current has to be increased.
Semiconductors such as silicon are known to have the property that the electrical resistance thereof decreases with increasing temperature.
Since the temperature in the interior of a heated rod is higher than at the surface thereof, which is cooled by the reaction gas, the electrical resistance in the interior of the rod and of the bridge is lower. Thus, the current flow in the interior of the bridge is higher.
In the limiting case of a high thermal flow due to significant cooling of the surface of the bridge by the reaction gases, this can lead to a temperature in the interior of the bridge which is above the melting point of silicon (1413° C.). This results in what is called “bridge leakage”, which leads inevitably to a stoppage of the deposition process.
DE 10 2007 047 210 A1 describes a process in which the probability of the bridge leakage is significantly increased.
This could be prevented only by reduction of the bridge temperature, which, however, would again nullify the advantages of the comparatively high deposition rate and of the improved energy efficiency.
DE 10 2007 023 041 A1 describes a further process for production of polysilicon, specifically for FZ (float zone) silicon. It envisages, up to a rod diameter of 30 mm, a rod temperature of 950 to 1090° C. and a particular proportion of chlorosilanes in the reaction gas, and, no later than after attainment of a rod diameter of 120 mm, switching of the rod temperature to 930 to 1030° C. and increasing the proportion of chlorosilanes in the reaction gas. Abrupt changes in the growth conditions must not be made over the entire deposition time.
Rods of polysilicon which are used for production of FZ silicon are brought to the desired diameter typically by means of mechanical processing after the deposition. In order to minimize the loss of silicon, all the rods produced should have the same diameter over the entire length. In addition, the cross section of the rods should be round over the entire length.
However, the rods of polysilicon produced according to DE 10 2007 023 041 A1 exhibit a geometry which is insufficiently constant with regard to the diameter as a function of the length of the crystals. The diameter varies with the length of the crystal, which has the result that more material has to be removed on one side in order to obtain the nominal diameter after mechanical processing. This reduces the economic viability of the process.
A further problem which often occurs in the prior art is dust deposition.
Dust deposition is referred to when the silicon-containing gas is not deposited at the surface of the rods (heterogeneous deposition), but rather reacts to give silicon in the free volume (homogeneous deposition).
The dust thus formed is firstly found at the base of the reactor at the end of the deposition process and has to be disposed of at a later stage.
Secondly, it is transported with the offgas to the offgas processing, where it can cause disruption.
Severe dust deposition can force stoppage of the deposition process. This reduces the economic viability.
In addition, it causes considerable problems in the industrial plants and associated additional cost and inconvenience.
Unfortunately, it is found that specifically deposition processes with a particularly high deposition rate lead in some cases to increased dust deposition.
Overall, it has not been possible in the prior art to date to harmonize the different aspects which are important in the deposition of polysilicon.
This problem gave rise to the objective of the present invention.